Rhodium nitrate is basically employed in two major catalytic processes. Approximately 72% of the present production of rhodium as rhodium nitrate is used as an impregnation agent for the manufacture of heterogeneous 3-way automotive exhaust catalyst. This catalyst is cordierite monolith coated with an aluminum oxide (Al.sub.2 O.sub.3) washcoat wherein the washcoat contains platinum, rhodium and palladium in reduced state. See U.S. Pat. No. 3,565,830.
Approximately 10-15% of the rhodium as rhodium nitrate ends up as a precursor in the manufacture of a homogeneous catalyst used in the well-known oxo alcohol process. See Organic Synthesis with Noble Metal Catalysts, P. N. Rylander, pages 60-76 (1973). This catalyst catalyzes the hydroformulation reaction in the oxo alcohol synthesis and is known as hydriodomethyltrisphenylphosphene rhodium-1. See Chemistry of Rarer Platinum Metals, W. P. Griffin, pages 370-388 (1967).
Also, some rhodium nitrate is used as a catalyst for the telomerization of butadiene with allyl alcohol, the polymerization of isoprene in allyl alcohol solution and the polymerization of butadiene. See Gmellin Handbook of Inorganic Chemistry, Rhodium, Supplemental Volume B1 (8th edition 1982), pages 65-66.
In order for rhodium nitrate to be stable and effective, it is essential that it be free of halide. Otherwise, even minor amount of the halide adversely effects its catalytic activity and performance. See U.S. Pat. No. 4,844,879. As described in said patent, rhodium nitrate is conventionally prepared by adding a caustic soda aqueous solution or aqueous ammonia to a rhodium chloride solution to first form rhodium hydroxide precipitate. The resulting rhodium hydroxide precipitate is then dissolved in nitric acid to produce rhodium nitrate. However, as is stated in said patent, the conversion of the rhodium chloride to rhodium hydroxide is insufficient, i.e., it is not quantitative, and hence the rhodium hydroxide will contain impurities (chloride) which ends up in the rhodium nitrate, and thus lowers its catalytic activity or efficiency. Accordingly, said patent describes a process for preparing a rhodium nitrate solution which comprises dissolving rhodium black in sulfuric acid, heating the reaction mixture, adjusting the pH of the resulting solution to neutral or alkaline (7 to 9) by the addition of an alkaline solution, thereby precipitating the rhodium as rhodium hydroxide. The precipitated rhodium hydroxide is then separated and dissolved in nitric acid to form rhodium nitrate. However, the conversion of rhodium to rhodium hydroxide is not quantitative and hence some of the rhodium is lost in the reaction.
Another method of manufacturing rhodium nitrate is described in the Russian Journal of Inorganic Chemistry, Volume 8, Number 13 (1973), page 1737. According to the method described in said Journal, when rhodium iodide (RhI.sub.3) is treated with boiling nitric acid, the black iodide dissolves in the acid forming rhodium nitrate, with the liberation of violet iodine vapor and brown nitrogen oxide. The rhodium nitrate produced, after drying to constant mass at 105.degree.-110.degree. C., is a brown solid product which does not contain iodide. However, when aqueous rhodium nitrate is heated to remove the nitric acid, the rhodium nitrate decomposes to produce rhodium oxide which has no catalytic value.
Accordingly, it is an object of this invention to provide a process for the manufacture of rhodium nitrate which is essentially free from halides, notably chloride.
It is a further object of this invention to provide such halide-free rhodium nitrate by a process which is quantitative wherein the reactants are completely converted to the desired products.
It is also an object of this invention to provide a halide-free rhodium nitrate which is particularly useful as a catalyst with excellent stability and catalytic efficiency.
The foregoing and other objects and features of this invention will become more apparent from the ensuing detailed description thereof.